Heretofore, as this type of device there has been such device as shown in FIGS. 1 to 3. In the figures, the numeral 1 is a first servomotor; 2 is a second servomotor; 3 is a third servomotor; 4 is a pulley which is fixed to an output shaft of the first servomotor 1; 5 is a belt for transmitting the rotation of the pulley 4 to a pulley 6; 7 is a box member; 8 is a pulley fixed to an output shaft of the second servomotor 2; 9 is a belt for transmitting the rotation of the pulley 8 to a pulley 10; 62 is a box member; 11 is a pulley fixed to an output shaft of the third servomotor 3; 12 is a belt for transmitting the rotation of the pulley 11 to a pulley 13; 63 is a box member; 14 and 16 are bevel gears which are fixed to the pulleys 6 and 10, respectively; 15 and 17 are bevel gears which are in mesh with the gears 14 and 16, respectively; 18, 19 and 20, 21 are bearings which support the pulley 6, bevel gear 14 and pulley 10, bevel gear 16, respectively; 26, 27 and 28 are drive shafts for transmission of the rotations of the bevel gear 15, bevel gear 17 and pulley 13, respectively; 22, 23 and 33 are bearings which support the drive shaft 28; 24, 32 and 25, 34 are bearings which support the drive shafts 26 and 27, respectively; 31 is a second link which encloses the drive shafts 26-28, etc.; 29 and 30 are bearings which support the second link 31; 64 is a first link which supports the second link 31 at one end thereof; 65 is a first box member fixed to a below-described internal gear 37; 35 is a bearing which supports the first box member 65 rotatably; 36 is a spur gear fixed to the drive shaft 26; 37 is an internal gear which is in mesh with the spur gear 36; 38 is a spur gear fixed to the drive shaft 27; 39 is a cylindrical spur gear which is in mesh with the spur gear 38; 40 is a bevel gear fixed to the spur gear 39; 41 and 42 are bearings which support the spur gear 39 and bevel gear 40; 43 is a bevel gear which is in mesh with the bevel gear 40; 48 is a pulley fixed to a boss portion of the bevel gear 43; 44 and 45 are bearings which support the bevel gear 43 and pulley 48; 46 is a bevel gear fixed to the drive shaft 28; 47 is a bevel gear which is in mesh with the bevel gear 46; 49 is a belt for transmitting the rotation of the pulley 48 to a pulley 50; 51 is a pulley fixed to a boss portion of the bevel gear 47; 52 is a belt for transmitting the rotation of the pulley 51 to a pulley 53; 66 is a second box member fixed to a boss portion of the pulley 50; 56 and 57 are bearings which support the pulley 50 and second box member 66 rotatably; 58 is a bevel gear fixed to the pulley 53; 59 is a bevel gear which is in mesh with the bevel gear 58, 59a is an output shaft constituting an output member which is fixed to the bevel gear 59; 60 and 61 are bearings which support the bevel gear 59 and output shaft 59a rotatably; 54 and 55 are bearings which support the pulley 53 and bevel gear 58; and 67 is a bearing which supports the box member 66. The spur gear 36, internal gear 37, first box member 65, etc. constitute a first device; the spur gears 38, 39, bevel gears 40, 43, pulleys 48, 50, second box member 66, etc. constitute a second device; the bevel gears 46, 47, pulleys 51, 53, bevel gears 58, 59, output shaft 59a, etc. constitute a third device.
The axis of rotation of the first box member 65 of the first device and that of the second box member 66 of the second device are orthogonal to each other, and the axis of rotation of the second box member 66 and that of the output shaft 59a of the third device are orthogonal to each other. The numeral 68 is a fourth servomotor with a reduction unit for driving the second link 31 through an eighth link 70 and a fourth link 71; 69 is a fifth servomotor with a reduction unit for driving the first link 64 on the side opposite to the fourth servomotor 68; and 72 is a pedestal which supports the first link 64 rotatably.
Operation will now be explained. The rotation of the first servomotor 1 is transmitted to the drive shaft 26 while undergoing deceleration and axial change successively through pulleys 4, 6 and bevel gears 14, 15, then slowed down by the spur gear 36 and internal gear 37 and causes the first box member 65 to rotate, whereby the operation of the first device is performed. Next, the rotation of the second servomotor 2 is transmitted to the drive shaft 27 while undergoing deceleration and axial change successively through pulleys 8, 10 and bevel gears 16, 17, then undergoes further deceleration and axial change through the spur gears 38, 39 and bevel gears 40, 43, and causes the second box member 66 to rotate through pulleys 48 and 50, whereby the operation of the second device is performed. The rotation of the third servomotor 3 is slowed down through pulleys 11 and 13 and transmitted to the drive shaft 28, then undergoes deceleration and axial change successively through bevel gears 46, 47, pulleys 51, 53 and bevel gears 58, 59, and causes the output shaft 59a to rotate, whereby the operation of the third device is performed.
The first and second devices, and the second and third devices, rotate about intersecting axes respectively. With the first to third devices, a work (not shown) secured to the output shaft 59a performs a so-called three-freedom wrist operation. Further, the rotation of the fourth servomotor causes the second link 31 to rotate through the fourth link 71 and third link 70, and the fifth servomotor 69 rotates the first link 64. The industrial robot combines the operations of the second link 31 and first link 64 which are performed by the fourth and fifth servomotors 68 and 69 and associated drive systems, with the above wrist operation and effects a five-freedom operation.
The first device which comprises the spur gear 36, internal gear 37, first box member 65, etc. is orthogonal to the axis of the second device which comprises the spur gears 38, 39, bevel gears 40, 43, pulleys 48, 50, second box member 66, etc. The second device is orthogonal to the axis of the third device which comprises the bevel gears 46, 47, pulleys 51, 53, bevel gears 58, 59, output shaft 59a, etc. For effecting the three-freedom wrist operation, it is necessary to use a large number of pulleys, spur gears, bevel gears and bearings. As a result, not only the number of components increases, but also the accuracy may be reduced by an accumulated error of each component. Besides, the wrist portion becomes more complicated and its weight increases, thus requiring increase in strength of each link which supports the wrist portion, and hence an increase in size of the system.
FIG. 4 is a sectional view of another conventional device which is disclosed, for example, in Japanese Patent Laid Open Publication No. 105463/77. In the figure, the numeral 31 is a cylindrical link; 28 is a first drive shaft disposed within the link 31; 27 is a cylindrical, second drive shaft which concentrically encloses the first drive shaft 28; 26 is a cylindrical, third drive shaft which concentrically encloses the second drive shaft 27; 65 is a first box member coupled to one end of the third drive shaft 26 and adapted to rotate concentrically therewith, one end of the first box member 65 being formed with a side plate 65b having a central support portion 65a which is located on an axis obliquely intersecting the axis of rotation of the first box member 65. The numeral 66 is a second box member having a cylindrical portion 66a which is rotatably supported by the support portion 65a, one end of the second box member 66 being formed with a support portion 66b which is located on an axis obliquely intersecting the axis of rotation of the second box member 66. The numeral 40 is a gear coupled to the second drive shaft 27; 49A is a gear shaf which is supported by the first box member 65 and which is in mesh with the gear 40; 43 is a gear which is in mesh with the gear shaft 49A and coupled to the cylindrical portion 66a of the second box member 66; 46 is a gear coupled to the first drive shaft 28; 47 is a gear which is in mesh with the gear 46 and concentric with the cylindrical portion 66a; 52A is a transmission shaft which is coupled to the gear 47 at one end thereof and disposed concentrically within the cylindrical portion 66a; 58 is a gear coupled to one end of the transmission shaft 52A; 59 is an output gear which is located on an axis obliquely intersecting the transmission shaft 52A and which is in mesh with the gear 58; and 100 is an output member such as a gripper or the like which is coupled to the output gear 59 and supported rotatably by the support portion 66b.
In the above construction, first, with rotation of the third drive shaft 26, the first box member 65 rotates about the axis of rotation, .alpha., of the third drive shaft 26. Next, with rotation of the second drive shaft 27, the second box member 66 rotates about the axis of rotation, .beta., via gear 40.fwdarw.gear shaft 49 A.fwdarw.gear 43.fwdarw.cylindrical portion 66a. Then, with rotation of the first drive shaft 28, the output member 100 rotates about the axis of rotation, .gamma., via gear 46.fwdarw.gear 47.fwdarw.transmission shaft 52A.fwdarw.gear 58.fwdarw.output gear 59. In this way, the output member 100, namely the gripper, performs a three-dimensional operation. For rotation of the second box member 66, a large rotational force is generally required in comparison with the output member 100. Therefore, the output member 100 is rotated by the first drive shaft 28 of a smaller diameter as well as the transmission shaft 52A, and the second box member 66 is rotated by the second drive shaft 27 of a larger diameter as well as the cylindrical portion 66a.
However, since the gear 43 is positioned on the side of an intersecting point of the axes .alpha. and .beta. with respect to the gear 47, the gears 40 and 43 are interconnected through the gear shaft 49A which constitutes an intermediate gear not contributing to deceleration. At a reduction ratio of the gears 40 and 43 it is impossible to expect a large increase in torque. Therefore, it is necessary that the input side of the second drive shaft 27 be set at a high torque in advance. This high torque is transmitted to the cylindrical portion 66a, namely the second box member 66, through the second drive shaft 27, gear 40, gear shaft 49A and gear 43. Accordingly, the strength of this transmission system must be set high with the result that not only the diameter of each shaft, gear tooth thickness, etc. increase, but also a rotational backlash of the first box member 65 relative to the link, that of the second box member 66 relative to the first box member 65 and that of the output member relative to the second box member 66 become larger due to accumulation of backlash of each gear, thus leading to deterioration in the operation accuracy of the output member 100.
Moreover, since the second box member is supported only at its cylindrical portion by the first box member, deflections would occur in the vicinity of the output member. Also in this respect, a bad influence is exerted on the operation accuracy of the output member.
As a still another conventional device of this type there is, for example, such device as disclosed in Japanese Patent Laid Open Publication No. 83265/78. This device is provided with three concentric shafts, to one of which is coupled a first box member adapted to rotate concentrically therewith. Supported rotatably by the first box member is a second box member, which is driven directly by a bevel gear coupled to another shaft in the triple shaft. More particularly, the second box member is directly formed with a gear which is in mesh with the above bevel gear, so it is rotated by the above bevel gear directly without intervention of an intermediate transmission shaft. By the way, as well known, the industrial robot is used for assembling, welding and like operations, for which a high operation accuracy is required. In assembling, therefore, the engagement of bevel gears should be adjusted. But, in a device in which a bevel gear is coupled to a second box member, the second box member itself must be handled together with the bevel gear for adjusting the engagement of the latter, thus resulting in a reduced working efficiency and need of much labor.